1. Field of the Invention
The present invention relates to methods and apparatus for evaluating velocity models and more particularly methods and apparatus for quantitatively determining the accuracy of velocity selections in converting arrival times to corresponding depths.
2. Related Prior Art
Good velocity model is the key to a successful pre-stack depth migration. At the present time, there is no rule of thumb by which the quality of the model can be quantitatively evaluated. Therefore, one cannot easily tell a good model from a poor model.
Migration techniques normally operate by broadcasting all recording events on a pre-stack input trace to all possible subsurface locations from which the reflection event could have originated.
When more traces are broadcasted, images begin to appear at places where broadcasted events are reinforcing each other. At places where no images are expected, the corresponding broadcasted events should cancel each other.
Prior art has disclosed many methods for processing seismic data which are used with common reflection point (CRP) gathers. Normal moveout correction is primarily used to compensate for noise or undesirable effects. As indicated previously, a significant problem with processing and migrating seismic data in the application of CRP analysis occurs when strong events occur near weak events. Examples of processing methods which include migration and normal moveout correction are as follows.
U.S. Pat. No. 4,813,027 titled "Method and Apparatus for Enhancing Seismic Data" (Hans Tieman) relates to a method and apparatus for stacking a plurality of seismic midpoint gathers to provide a pictorial representation of seismic events. The approximate propagation velocity, corresponding to a selected event in a common midpoint gather, is determined by summing the common midpoint gather using first and second weights to provide respective first and second weighted sums over an offset based on an estimated velocity corresponding to the event. A velocity error value indicative of the approximate error between the estimated velocity and the actual velocity is developed from the sums. The common midpoint gather is then restacked in accordance with the determined propagation velocity to provide an enhanced pictorial representation of the seismic event. The first and second weighted sums are taken over a time window centered upon an estimated zero offset travel time for the event. The first and second weights can be selected to provide rapid, slow or intermediate convergence upon the true velocity. The velocity error value is determined as a function of the deviation of the peak of the first weighted sum from the center of the time window, relative to the deviation of the peak of the second weighted sum from the center of the time window. Alternatively, the velocity error value is determined as a function of the deviation of the peak of the cross-correlation of the first and second weighted sums from the center of the time window.
U.S. Pat. No. 4,241,429 titled "Velocity Determination and Stacking Process from Seismic Exploration of Three Dimensional Reflection Geometry" (Marvin G. Bloomquist et al.) relates to a method for determining the dip and strike of subsurface interfaces and average propagation velocity of seismic waves. In seismic exploration, linear, multiple fold, common depth point sets of seismograms with three dimensional reflection geometry are used to determine the dip and strike of the subsurface reflecting interfaces and the average velocity of the path of the seismic energy to the reflecting interface. The reflections in each set appear with time differences on a hyperbola with trace spacings determined by the source receiver coordinate distance along the lines of exploration. The offset of the apex of this hyperbola is determined from a normal moveout velocity search of the type performed on two dimensional common depth point (CDP) sets. This search identifies the correct stacking velocity and hyperbola offset which are used to determine dip, strike and average velocity.
U.S. Pat. No. 4,802,146 titled "Method for Moveout Correction and Stacking Velocity Estimation of Offset VSP Data" (George P. Moeckel) relates to a moveout correction process and stacking velocity estimation process to permit stacking of vertical seismic profile (VSP) data. The primary reflection time is determined by using the two-way travel time, the root mean square velocity of acoustic pulses in the formation and the first arrival time of direct path acoustic pulses.
U.S. Pat. No. 4,736,347 titled "Multiple Stacking and Spatial Mapping of Seismic Data" (Bernard Goldberg et al.) relates to a method for determining the dip of subsurface formations and the apparent acoustic velocity. Seismic traces are stacked in a plurality of orthogonal measures to form multiple stacked traces at a positive offset. The stacking process determines the apparent velocities as functions of the travel time at the positive offset. The interval acoustic velocity of the first layer is then determined from knowledge of surface topography, source-receiver offset, two-way travel times and the first reflector apparent velocities. The first layer velocity information enables the incident and emergent angles of the raypaths at the surface to be calculated, as well as enabling the dip angles and spatial coordinates of the reflection points on the first reflecting boundary to be determined. Seismic data corresponding to the second reflecting boundary are then mapped spatially to the first reflecting boundary by ray tracing and by calculating the apparent velocities at the first boundary. The process is repeated for each succeedingly deeper boundary. The derived acoustic velocity model of the earth is displayed as a stacked seismic section in spatial coordinates. This process may be applied to obtain earth models and seismic sections in both two and three dimensions.
Although prior art has shown many methods for determining velocity through subsurface layers, there has been no accurate quantitative method for determining subsurface velocities.